Currently, in the field of security systems, motion detectors are generally provided to detect intruders. Dual technology motion detectors incorporate multiple sensing technologies in order to provide efficient intruder detection with minimized risk of false alarms. Many dual detectors incorporate passive infrared (PIR) technology and microwave (MW) technology.
PIR technology has long been used in motion detectors. The PIR sensor detects the difference between the infrared energy emitted from an intruder and that emitted from the ambient environment. Many PIR detectors utilize Fresnel lenses or custom shaped mirrors to focus infrared energy on a pyrodetector. The output signal from the pyrodetector is then processed via analog hardware and/or digital signal processing. Lenses and mirrors are designed to provide various detection zones emanating radially from the sensor. As a target moves across the PIR detection zones, the sensing elements within the pyrodetector are alternately exposed to the target IF energy, resulting in an alternating voltage output from the PIR sensor. The amplitude and frequency of this voltage vary with a number of factors including target size, speed, and direction relative to the PIR zones, difference between ambient and target temperature, width and spacing between the detection zones, and frequency response of the pyrodetector.
Upon receiving the signals, the detector may perform processing by comparing the received signal to one or more voltage thresholds. These threshold crossings produce positive and negative pulses that can be counted and timed, with certain combinations of pulse height, duration, and frequency being considered PIR alarms.
MW technology often operates on the principle of phase shift or Doppler effect. Unlike PIR, MW technology is an active technology. The MW detector transmits MW energy, which reflects off objects and returns to the MW detector. Moving objects result in a received signal that is frequency shifted from the original transmitted signal. The detector receives this signal, and generates an alternating voltage difference frequency signal which is then processed via hardware or digital signal processing. Because only the AC (alternating current) component of the signal is processed, only moving objects are detected. The frequency of the returned signal is dependent upon the target movement direction and speed. The amplitude is a function of transmitted signal strength, target size, distance, and reflectivity. Highly reflective materials such as metal will return significantly greater energy to the detector than less reflective objects such as people or animals. Processing may include comparison of the MW signal to one or more thresholds with certain combinations of quantity, duration, or frequency of threshold crossings considered MW alarms.
Typically, dual technology detectors incorporate an AND function, whereby both individual sensor technologies must be in the alarm state simultaneously to produce an alarm at the detector output. Thus, if either the MW or PIR detectors are disabled, the system will fail to generate an alarm. PIR motion detectors are susceptible to being blocked or disabled. Generally, if the PIR motion detector is blocked, the dual detector system fails to recognize the blockage and will thus also fail to alarm. A system is needed for recognizing this problem and allowing the dual detector system to generate an alarm and/or provide notification that the range of the detector has been reduced.
Some currently existing systems, such as that of U.S. Patent Application No. 2004/0160324 disclose the use of a dual detector system in which a technique is provided for determining if a PIR sensor may be blocked. However, this system does not automatically make this determination and provide output based on this determination. Accordingly, a solution is need that automatically detects PIR detector blockage and makes alarm decisions, and/or provides trouble indication based on this determination.